Anthracene, a component of coal tar, has proven to be a very potent photosensitizer in human erythrocyte ghosts. Anthracene was bound covalently to different positions on fatty acid molecules. Phototoxicity studies showed that when anthracene was bound to the polar end, it was more photoxic than anthracene alone. However when bound to the apolar end, anthracene had little phototoxic effect. The major site of anthracene phototoxic reactions therefore appear to be close to the protein-lipid interface within the ghost membranes.